An acrylic rubber is superior in heat resistance, oil resistance, etc., so is being widely used in the automobile-related field etc. for hoses, seals, gaskets, and other rubber members.
On the other hand, such automobile-use rubber members, in particular, rubber members in the engine compartment, are being asked to provide further improved heat resistance performance due to the higher performance of turbo charger accompanying the higher output of engines and recent tougher exhaust gas regulations etc.
As a method for improving the heat resistance of rubber members, in the past, blending an antiaging agent into the acrylic rubber has been studied. As such an antiaging agent, diphenylamine-based antiaging agents etc. have been used. Among diphenylamine-based antiaging agents as well, 4,4′-bis(α,α-dimethylbenzyl)diphenylamine is known to have a high heat resistance effect. However, the rubber which has blended 4,4′-bis(α,α-dimethylbenzyl)diphenylamine which has been used in the past alone, cannot satisfy the recent demands for heat resistance. For this reason, various proposals are being made for further improving the heat resistance.
For example, Patent Document 1 discloses an acrylic rubber composition which is comprised of an acrylic rubber into which two types of diphenylamine-based antiaging agents are jointly blended. However, while the art which is described in Patent Document 1 improves the rate of change of tensile strength, rate of change of elongation, and rate of compression set in relatively short term heat resistance tests, the effect of improvement has been insufficient in high temperature, long term heat resistance tests.
Patent Document 2 discloses the art of using, as the antiaging agent which is blended into the acrylic rubber, instead of 4,4′-bis(α,α-dimethylbenzyl)diphenylamine, a specific styrenated diphenylamine compound. Further, Patent Document 3 discloses an acrylic rubber composition which comprises a carboxyl group-containing acrylic rubber into which a secondary diamine-based antiaging agent and a secondary monoamine-based antiaging agent are jointly blended. However, while the arts which are described in Patent Document 2 and Patent Document 3 improve the rate of change of tensile strength in a relatively short term heat resistance test compared with using an antiaging agent constituted by 4,4′-bis(α,α-dimethylbenzyl)diphenylamine alone, the effect of improvement of the rate of change of elongation or the rate of compression set was insufficient.
Furthermore, Patent Document 4 discloses a cross-linkable acrylic rubber composition which is comprised of acrylic rubber into which a polyvalent primary amine cross-linking agent and an antiaging agent constituted by p-amino diphenylamine are blended. The art which is described in Patent Document 4 improves the rate of change of elongation in a heat resistance test compared with the case of using an antiaging agent constituted by 4,4′-bis(α,α-dimethylbenzyl)diphenylamine alone. However, to sufficiently meet the recent demands for heat resistance for acrylic rubber, further improvement has been desired.